1. Field of the Invention
The present invention relates to a device for controlling an internal combustion engine.
2. Related Art
In the past, there has been known an internal combustion engine designed to find the amount of stored oxygen of a three-way catalyst arranged in the engine exhaust passage, switch the air-fuel ratio just slightly to lean when the amount of stored oxygen of the three-way catalyst becomes smaller, switch the air-fuel ratio just slightly to rich when the amount of stored oxygen of the three-way catalyst becomes greater, and thereby maintain the amount of stored oxygen of the three-way catalyst in the target range. This will be explained later in more detail, but if doing this, it is possible to efficiently purify the unburned HC, CO, and NOx contained in the exhaust gas even when the air-fuel ratio greatly deviates from the stoichiometric air-fuel ratio.
To maintain the amount of stored oxygen of the three-way catalyst in the target range, it is necessary to accurately find the amount of stored oxygen of the three-way catalyst. On the other hand, at the time of engine deceleration operation, the general practice has been to temporarily cut the fuel to the engine. By cutting the fuel to the engine, air passes through the combustion chambers and directly flows into the three-way catalyst.
Therefore, there is known an internal combustion engine designed to calculate the amount of stored oxygen of the three-way catalyst when the fuel to the engine is cut by adding a relatively large addition term, calculate the amount of stored oxygen of the three-way catalyst when the fuel to the engine is not cut and the air-fuel ratio is rich by subtracting a relatively small subtraction term, and calculate the amount of stored oxygen of the three-way catalyst when the fuel to the engine is not cut and the air-fuel ratio is lean by adding a relatively small addition term (see Japanese Unexamined Patent Publication (Kokai) No. 2001-227383).
However, when an additional three-way catalyst is arranged in the exhaust passage upstream of that catalyst, there is the problem that what is calculated by the above-mentioned method is the total of the amount of stored oxygen of the two three-way catalysts or the amount of stored oxygen of the additional three-way catalyst at the upstream-side and therefore the amount of stored oxygen of the downstream-side three-way catalyst cannot be calculated independently.